The generation of ions of heavy analyte molecules with molecular weights of several hundred to many thousand daltons in an electrospray ion source is known. The ability to ionize macromolecules, which cannot be vaporized thermally, is extremely important. For example, in 2002, John Bennett Fenn was awarded a Nobel Prize for Chemistry for the development of the electrospray ion source toward the end of the 1980s.
A high voltage of several kilovolts is applied to a long, pointed spray capillary containing spray liquid with dissolved analyte molecules to generate an extremely strong electric field about the tip. This polarizes the surface of the spray liquid in the open tip and strongly charges it; the electrostatic pulling force fauns a so-called Taylor cone on the surface of the liquid, and the electric drawing field draws a fine jet of liquid out of the tip of the cone. This jet is intrinsically unstable due to its high surface charge, which opposes the surface tension. The jet disintegrates by constriction into minute, highly charged droplets with diameters in the order of one micrometer. Droplets with such a diameter carry about 50,000 elementary charges. The decomposition into droplets can be assisted by a sharp jet of a spray gas blown in around the tip of the capillary by a concentric spray gas capillary. This causes the jet of droplets to be guided in a somewhat more focused way, although the resulting droplets have greater diameter variance.
The droplets subsequently evaporate in a hot drying gas, a process whereby predominantly only neutral solvent molecules vaporize. This causes the charge density on the surface to continuously increase. When the density of the charges on the surface becomes so large that the Coulomb repulsion exceeds the force of the cohesive surface tension (“Rayleigh limit”), small droplets start to split off. The unstable surface brings about random deforming oscillations of the fluid on the surface, and these random motions, in turn, cause smaller droplets to separate off, causing the charges of both droplets to fall below the Rayleigh limit again. The smaller droplets which split off usually have a much higher charge in relation to their mass. This is possible because the total charge quantity q of a droplet at the Rayleigh limit is proportional to the root of the third power √d3=d1.5 of the diameter d. Split-off droplets can thus only carry away two percent of the mass, but fifteen percent of the charge, for example. However, the droplets generated, both large and small, have a mass-to-charge ratio above the Rayleigh limit, and can thus vaporize further.
All droplets, large and small, continue to vaporize, small droplets vaporizing faster and faster due to the fact that the coordination number of their surface molecules gets ever smaller, causing the vapor pressure to increase, until the splitting off and vaporization processes end relatively rapidly in a complete drying of a droplet, and multiply charged ions of the analyte molecules contained in the droplet remain. These analyte ions are generally only surrounded by a relatively strongly bound sheath of one to two molecular layers of polar solvent molecules, usually water.
The objective for any electrospray ion source is to draw in as many of these analyte ions as possible, together with hot drying gas into the inlet aperture of an ion analyzer operating in vacuum, for example a mass spectrometer or ion mobility spectrometer. Usually, the inlet aperture belongs to an inlet capillary. The analyte ions lose most of their solvate sheath on their way through the inlet capillary into the vacuum system, a process which is assisted by a heated drying gas and the decreasing pressure along the inlet capillary. In the ion analyzer, the analyte ions undergo the desired type of analysis. It is also possible to bundle several inlet capillaries in order to introduce the analyte ions into the vacuum; this bundle of inlet capillaries shall be included here when the term “inlet capillary” is used. On the other hand, an inlet capillary may be just a fine aperture.
The analyte ions generated by electro spraying are predominantly multiply charged; the number of charges for ions of a single substance varies greatly, however, and the average number of charges increases roughly in proportion to the mass of the analyte ions. The mass-to-charge ratios m/z (m=mass; z=number of excess elementary charges of the ion) have a broad distribution for heavy ions from about m/z=700 daltons up to about m/z=1,600 daltons, with a maximum at around m/z=1,200 daltons. Thus the heavy molecules of albumin (m=66 kDa) carry about 50 charges on average, while light molecules with molecular weights below m=1 kDa are predominantly singly charged. The distribution of the charges can be affected by the composition of the solvent, the spraying processes and the guidance of the ions through gases.
Since the droplets of the spray jet from the spray capillary are all highly charged, with 50,000 elementary charges per droplet, for example, they strongly repel each other. This causes the spray mist with the spray droplets, which are at first accelerated in the electric field, to broaden into a trumpet shape immediately after the droplets have been formed, at least in the case where no spray gas is applied. The space where the analyte ions are located after the liquid has vaporized from the droplets is thus extremely expanded. This spatial region will be called “ion formation space” below. It is difficult to draw a large number of analyte ions from a large spatial region into the inlet capillary.
A spray gas supplied in a sharp jet, which can be heated up to around 150° C., can be used to reduce the broadening of the spray mist, but the spray droplets are also accelerated. However, this produces an elongated ion formation space of moderate width, in which many fast, unvaporized droplets fly through the cloud of analyte ions. Since the inlet capillary must preferably not draw in any droplets, an additional problem involves keeping the droplets away from the entrance of the inlet capillary. This problem has led to the development of spray devices positioned orthogonally to the inlet capillary with spray gas blown in sharply to shoot the larger droplets way past the inlet capillary and into a waste aperture. This means that the analyte molecules in the larger droplets are lost to the analyses; the sensitivity of the method in principle is thus decreased.
The analyte ions produced in the greatly elongated ion formation space of moderate width, when spray gas is used, are extracted in the perpendicular direction and fed to the inlet capillary. This is successful for only a small portion of the analyte ions, because only analyte ions from a section of the length and width of this ion formation space reach the inlet capillary. More analyte ions can be extracted if the focusing of the ion formation space in the axial direction can be improved. This can be achieved by blowing in a “super hot” sheath gas at approx. 300° C. around the hot spray gas. The droplets are then “thermally focused”; the utilization of the analyte ions is better and the sensitivity of the method is greater.